Crystal Structure Controls Light-Driven Spin Currents, Opening Path to Spintronic Devices

A team of researchers at Tokyo Institute of Science has solved a puzzle that has long complicated the design of spintronic devices: where exactly does the photocurrent come from when circularly polarized light strikes certain materials? The answer, published in September 2026 in Nano Letters, is that it originates from deep within the crystal's structure itself, not from its surface—a distinction that opens new possibilities for controlling how light generates spin-dependent electrical currents.

The phenomenon at the center of this work is called the circular photogalvanic effect, or CPGE. When circularly polarized light—light that spirals either left or right as it travels—hits certain materials, it produces an electrical current whose direction depends on the handedness of that light. This effect has long attracted attention from scientists working on spintronics, a field that seeks to harness electron spin rather than electron charge to build smaller, more efficient electronic devices. The appeal is straightforward: if you can generate and control spin-polarized currents using nothing but light, you have a powerful tool for building the next generation of information technology.

But there has been a persistent problem. When researchers measure CPGE in real materials, they cannot easily tell whether the photocurrent they are detecting comes from the bulk of the material or from its surface and interfaces. These two sources produce different signals, and they get mixed together in conventional measurements, making it nearly impossible to understand which part of the material is actually responsible for the effect. Professor Kouji Taniguchi and his team at Tokyo Institute of Science, including graduate student Ichi Naruse and Assistant Professor Po-Jung Huang, set out to untangle this confusion.

They worked with two-dimensional organic-inorganic hybrid perovskites—materials made of alternating layers of lead iodide and organic cations. The lead atoms create strong spin-orbit interactions, the quantum mechanical coupling between electron motion and spin that makes these materials useful for spintronic applications. The organic molecules, meanwhile, carry permanent electric dipoles that give the entire crystal a built-in polarity, like a permanent magnet. The researchers exposed these crystals to circularly polarized light coming straight down at the surface, perpendicular to it. When they measured the resulting photocurrent in one direction, they found it reversed when they switched the light's handedness. But when they rotated their measurement electrodes by ninety degrees and looked for photocurrent in a perpendicular direction, they found nothing—exactly what the crystal's symmetry predicted if the effect came from the bulk. This was the key insight: the geometry of normal incidence suppresses surface contributions while allowing the bulk signal to shine through.

To confirm this finding, the researchers took an additional step. They synthesized two versions of their crystals, one containing right-handed organic molecules and one containing left-handed versions—mirror images of each other, or enantiomers. The two crystals had opposite polarization directions built into their structure. When the team measured the CPGE photocurrent in each crystal, the sign reversed between them. This proved beyond doubt that the direction of the photocurrent was determined by the bulk polarity of the crystal, not by surface effects or any other confounding factor.

The implications are significant. By showing that molecular design can control the direction of light-driven photocurrents, the researchers have provided a blueprint for engineering materials with tailored spin-dependent properties. The work also offers a general method for distinguishing bulk from surface photoresponses in atomically thin hybrid materials—a distinction that will matter for anyone trying to design and optimize these systems. Taniguchi notes that the findings could accelerate development of helicity-sensitive photodetectors, spin-photonic devices, and next-generation opto-spintronic materials based on two-dimensional perovskites. The path from laboratory discovery to practical device is never short, but this work has removed a significant obstacle from the way.